Fyn, a novel human proto-oncogene of the src gene family, has been localized by in situ hybridization on chromosome 6. This localization shows that src genes are located on different chromosomes and represent distinct loci. Another proto-oncogene, dbl. isolated from a diffuse B-cell lymphoma, was localized on the X chromosome. Sex chromosomes, therefore, like autosomes, have oncogenic potential. A gene coding for a human cellular thyroid hormone binding protein (P55) was assigned on chromosome 17. The precise localization of these three genes will permit the evaluation of possible effects of chromosome changes on their structure and activity in malignancies and chromosome syndromes. Cellular sequences flanking an integrated human papillomavirus (HPV)-18 DNA copy was localized on chromosomes from normal and HeLa cells to chromosome 8 near the myc gene. The integration at this site was accompanied by amplification of viral sequences and increased myc RNA expression. These alterations may have contributed to the development of this malignancy.